With help from the C.I.A., Arab governments and Turkey have sharply increased their military aid to Syria’s opposition fighters in recent months, expanding a secret airlift of arms and equipment for the uprising against President Bashar al-Assad, according to air traffic data, interviews with officials in several countries and the accounts of rebel commanders.

The airlift, which began on a small scale in early 2012 and continued intermittently through last fall, expanded into a steady and much heavier flow late last year, the data shows. It has grown to include more than 160 military cargo flights by Jordanian, Saudi and Qatari military-style cargo planes landing at Esenboga Airport near Ankara, and, to a lesser degree, at other Turkish and Jordanian airports.

As it evolved, the airlift correlated with shifts in the war within Syria, as rebels drove Syria’s army from territory by the middle of last year. And even as the Obama administration has publicly refused to give more than “nonlethal” aid to the rebels, the involvement of the C.I.A. in the arms shipments — albeit mostly in a consultative role, American officials say — has shown that the United States is more willing to help its Arab allies support the lethal side of the civil war.

From offices at secret locations, American intelligence officers have helped the Arab governments shop for weapons, including a large procurement from Croatia, and have vetted rebel commanders and groups to determine who should receive the weapons as they arrive, according to American officials speaking on the condition of anonymity. The C.I.A. declined to comment on the shipments or its role in them.

The shipments also highlight the competition for Syria’s future between Sunni Muslim states and Iran, the Shiite theocracy that remains Mr. Assad’s main ally. Secretary of State John Kerry pressed Iraq on Sunday to do more to halt Iranian arms shipments through its airspace; he did so even as the most recent military cargo flight from Qatar for the rebels landed at Esenboga early Sunday night.

Syrian opposition figures and some American lawmakers and officials have argued that Russian and Iranian arms shipments to support Mr. Assad’s government have made arming the rebels more necessary.

Most of the cargo flights have occurred since November, after the presidential election in the United States and as the Turkish and Arab governments grew more frustrated by the rebels’ slow progress against Mr. Assad’s well-equipped military. The flights also became more frequent as the humanitarian crisis inside Syria deepened in the winter and cascades of refugees crossed into neighboring countries.

The Turkish government has had oversight over much of the program, down to affixing transponders to trucks ferrying the military goods through Turkey so it might monitor shipments as they move by land into Syria, officials said. The scale of shipments was very large, according to officials familiar with the pipeline and to an arms-trafficking investigator who assembled data on the cargo planes involved.

“A conservative estimate of the payload of these flights would be 3,500 tons of military equipment,” said Hugh Griffiths, of the Stockholm International Peace Research Institute, who monitors illicit arms transfers.

“The intensity and frequency of these flights,” he added, are “suggestive of a well-planned and coordinated clandestine military logistics operation.”

Although rebel commanders and the data indicate that Qatar and Saudi Arabia had been shipping military materials via Turkey to the opposition since early and late 2012, respectively, a major hurdle was removed late last fall after the Turkish government agreed to allow the pace of air shipments to accelerate, officials said.

Simultaneously, arms and equipment were being purchased by Saudi Arabia in Croatia and flown to Jordan on Jordanian cargo planes for rebels working in southern Syria and for retransfer to Turkey for rebels groups operating from there, several officials said.

These multiple logistics streams throughout the winter formed what one former American official who was briefed on the program called “a cataract of weaponry.”

American officials, rebel commanders and a Turkish opposition politician have described the Arab roles as an open secret, but have also said the program is freighted with risk, including the possibility of drawing Turkey or Jordan actively into the war and of provoking military action by Iran.

Still, rebel commanders have criticized the shipments as insufficient, saying the quantities of weapons they receive are too small and the types too light to fight Mr. Assad’s military effectively. They also accused those distributing the weapons of being parsimonious or corrupt.

“The outside countries give us weapons and bullets little by little,” said Abdel Rahman Ayachi, a commander in Soquor al-Sham, an Islamist fighting group in northern Syria.

He made a gesture as if switching on and off a tap. “They open and they close the way to the bullets like water,” he said.

Two other commanders, Hassan Aboud of Soquor al-Sham and Abu Ayman of Ahrar al-Sham, another Islamist group, said that whoever was vetting which groups receive the weapons was doing an inadequate job.

“There are fake Free Syrian Army brigades claiming to be revolutionaries, and when they get the weapons they sell them in trade,” Mr. Aboud said.

The former American official noted that the size of the shipments and the degree of distributions are voluminous.

“People hear the amounts flowing in, and it is huge,” he said. “But they burn through a million rounds of ammo in two weeks.”

A Tentative Start

The airlift to Syrian rebels began slowly. On Jan. 3, 2012, months after the crackdown by the Alawite-led government against antigovernment demonstrators had morphed into a military campaign, a pair of Qatar Emiri Air Force C-130 transport aircraft touched down in Istanbul, according to air traffic data.

They were a vanguard.

Weeks later, the Syrian Army besieged Homs, Syria’s third largest city. Artillery and tanks pounded neighborhoods. Ground forces moved in.

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Even with the weapons from abroad, rebels say they are hard pressed to fight the government.CreditAbdalghne Karoof/Reuters

Across the country, the army and loyalist militias were trying to stamp out the rebellion with force — further infuriating Syria’s Sunni Arab majority, which was severely outgunned. The rebels called for international help, and more weapons.

By late midspring the first stream of cargo flights from an Arab state began, according to air traffic data and information from plane spotters.

On a string of nights from April 26 through May 4, a Qatari Air Force C-17 — a huge American-made cargo plane — made six landings in Turkey, at Esenboga Airport. By Aug. 8 the Qataris had made 14 more cargo flights. All came from Al Udeid Air Base in Qatar, a hub for American military logistics in the Middle East.

Qatar has denied providing any arms to the rebels. A Qatari official, who requested anonymity, said Qatar has shipped in only what he called nonlethal aid. He declined to answer further questions. It is not clear whether Qatar has purchased and supplied the arms alone or is also providing air transportation service for other donors. But American and other Western officials, and rebel commanders, have said Qatar has been an active arms supplier — so much so that the United States became concerned about some of the Islamist groups that Qatar has armed.

The Qatari flights aligned with the tide-turning military campaign by rebel forces in the northern province of Idlib, as their campaign of ambushes, roadside bombs and attacks on isolated outposts began driving Mr. Assad’s military and supporting militias from parts of the countryside.

As flights continued into the summer, the rebels also opened an offensive in that city — a battle that soon bogged down.

The former American official said David H. Petraeus, the C.I.A. director until November, had been instrumental in helping to get this aviation network moving and had prodded various countries to work together on it. Mr. Petraeus did not return multiple e-mails asking for comment.

The American government became involved, the former American official said, in part because there was a sense that other states would arm the rebels anyhow. The C.I.A. role in facilitating the shipments, he said, gave the United States a degree of influence over the process, including trying to steer weapons away from Islamist groups and persuading donors to withhold portable antiaircraft missiles that might be used in future terrorist attacks on civilian aircraft.

American officials have confirmed that senior White House officials were regularly briefed on the shipments. “These countries were going to do it one way or another,” the former official said. “They weren’t asking for a ‘Mother, may I?’ from us. But if we could help them in certain ways, they’d appreciate that.”

Through the fall, the Qatari Air Force cargo fleet became even more busy, running flights almost every other day in October. But the rebels were clamoring for even more weapons, continuing to assert that they lacked the firepower to fight a military armed with tanks, artillery, multiple rocket launchers and aircraft.

Many were also complaining, saying they were hearing from arms donors that the Obama administration was limiting their supplies and blocking the distribution of the antiaircraft and anti-armor weapons they most sought. These complaints continue.

“Arming or not arming, lethal or nonlethal — it all depends on what America says,” said Mohammed Abu Ahmed, who leads a band of anti-Assad fighters in Idlib Province.

The Breakout

Soon, other players joined the airlift: In November, three Royal Jordanian Air Force C-130s landed in Esenboga, in a hint at what would become a stepped-up Jordanian and Saudi role.

Within three weeks, two other Jordanian cargo planes began making a round-trip run between Amman, the capital of Jordan, and Zagreb, the capital of Croatia, where, officials from several countries said, the aircraft were picking up a large Saudi purchase of infantry arms from a Croatian-controlled stockpile.

The first flight returned to Amman on Dec. 15, according to intercepts of a transponder from one of the aircraft recorded by a plane spotter in Cyprus and air traffic control data from an aviation official in the region.

In all, records show that two Jordanian Ilyushins bearing the logo of the Jordanian International Air Cargo firm but flying under Jordanian military call signs made a combined 36 round-trip flights between Amman and Croatia from December through February. The same two planes made five flights between Amman and Turkey this January.

As the Jordanian flights were under way, the Qatari flights continued and the Royal Saudi Air Force began a busy schedule, too — making at least 30 C-130 flights into Esenboga from mid-February to early March this year, according to flight data provided by a regional air traffic control official.

Several of the Saudi flights were spotted coming and going at Ankara by civilians, who alerted opposition politicians in Turkey.

“The use of Turkish airspace at such a critical time, with the conflict in Syria across our borders, and by foreign planes from countries that are known to be central to the conflict, defines Turkey as a party in the conflict,” said Attilla Kart, a member of the Turkish Parliament from the C.H.P. opposition party, who confirmed details about several Saudi shipments. “The government has the responsibility to respond to these claims.”

Turkish and Saudi Arabian officials declined to discuss the flights or any arms transfers. The Turkish government has not officially approved military aid to Syrian rebels.

Croatia and Jordan both denied any role in moving arms to the Syrian rebels. Jordanian aviation officials went so far as to insist that no cargo flights occurred.

The director of cargo for Jordanian International Air Cargo, Muhammad Jubour, insisted on March 7 that his firm had no knowledge of any flights to or from Croatia.

“This is all lies,” he said. “We never did any such thing.”

A regional air traffic official who has been researching the flights confirmed the flight data, and offered an explanation. “Jordanian International Air Cargo,” the official said, “is a front company for Jordan’s air force.”

After being informed of the air-traffic control and transponder data that showed the plane’s routes, Mr. Jubour, from the cargo company, claimed that his firm did not own any Ilyushin cargo planes.

Asked why his employer’s Web site still displayed images of two Ilyushin-76MFs and text claiming they were part of the company fleet, Mr. Jubour had no immediate reply. That night the company’s Web site was taken down.

Reporting was contributed by Robert F. Worth from Washington and Istanbul; Dan Bilefsky from Paris; and Sebnem Arsu from Istanbul and Ankara, Turkey.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Airlift To Rebels In Syria Expands With C.I.A.’S Help. Order Reprints | Today’s Paper | Subscribe